feat(ssa): Hoist MakeArray instructions during loop invariant code mo…

…tion (#6782)

Co-authored-by: Tom French <15848336+TomAFrench@users.noreply.github.com>

compiler/noirc_evaluator/src/ssa/opt/loop_invariant.rs

@@ -109,6 +109,22 @@ impl<'f> LoopInvariantContext<'f> {
 
                 if hoist_invariant {
                     self.inserter.push_instruction(instruction_id, pre_header);
+
+                    // If we are hoisting a MakeArray instruction,
+                    // we need to issue an extra inc_rc in case they are mutated afterward.
+                    if matches!(
+                        self.inserter.function.dfg[instruction_id],
+                        Instruction::MakeArray { .. }
+                    ) {
+                        let result =
+                            self.inserter.function.dfg.instruction_results(instruction_id)[0];
+                        let inc_rc = Instruction::IncrementRc { value: result };
+                        let call_stack = self.inserter.function.dfg.get_call_stack(instruction_id);
+                        self.inserter
+                            .function
+                            .dfg
+                            .insert_instruction_and_results(inc_rc, *block, None, call_stack);
+                    }
                 } else {
                     self.inserter.push_instruction(instruction_id, *block);
                 }
@@ -186,6 +202,7 @@ impl<'f> LoopInvariantContext<'f> {
         });
 
         let can_be_deduplicated = instruction.can_be_deduplicated(self.inserter.function, false)
+            || matches!(instruction, Instruction::MakeArray { .. })
             || self.can_be_deduplicated_from_upper_bound(&instruction);
 
         is_loop_invariant && can_be_deduplicated
@@ -555,4 +572,94 @@ mod test {
         let ssa = ssa.loop_invariant_code_motion();
         assert_normalized_ssa_equals(ssa, expected);
     }
+
+    #[test]
+    fn insert_inc_rc_when_moving_make_array() {
+        // SSA for the following program:
+        //
+        // unconstrained fn main(x: u32, y: u32) {
+        //   let mut a1 = [1, 2, 3, 4, 5];
+        //   a1[x] = 64;
+        //   for i in 0 .. 5 {
+        //       let mut a2 = [1, 2, 3, 4, 5];
+        //       a2[y + i] = 128;
+        //       foo(a2);
+        //   }
+        //   foo(a1);
+        // }
+        //
+        // We want to make sure move a loop invariant make_array instruction,
+        // to account for whether that array has been marked as mutable.
+        // To do so, we increment the reference counter on the array we are moving.
+        // In the SSA below, we want to move `v42` out of the loop.
+        let src = "
+        brillig(inline) fn main f0 {
+          b0(v0: u32, v1: u32):
+            v8 = make_array [Field 1, Field 2, Field 3, Field 4, Field 5] : [Field; 5]
+            v9 = allocate -> &mut [Field; 5]
+            v11 = array_set v8, index v0, value Field 64
+            v13 = add v0, u32 1
+            store v11 at v9
+            jmp b1(u32 0)
+          b1(v2: u32):
+            v16 = lt v2, u32 5
+            jmpif v16 then: b3, else: b2
+          b2():
+            v17 = load v9 -> [Field; 5]
+            call f1(v17)
+            return
+          b3():
+            v19 = make_array [Field 1, Field 2, Field 3, Field 4, Field 5] : [Field; 5]
+            v20 = allocate -> &mut [Field; 5]
+            v21 = add v1, v2
+            v23 = array_set v19, index v21, value Field 128
+            call f1(v23)
+            v25 = add v2, u32 1
+            jmp b1(v25)
+        }
+        brillig(inline) fn foo f1 {
+          b0(v0: [Field; 5]):
+            return
+        }
+        ";
+
+        let ssa = Ssa::from_str(src).unwrap();
+
+        // We expect the `make_array` at the top of `b3` to be replaced with an `inc_rc`
+        // of the newly hoisted `make_array` at the end of `b0`.
+        let expected = "
+        brillig(inline) fn main f0 {
+          b0(v0: u32, v1: u32):
+            v8 = make_array [Field 1, Field 2, Field 3, Field 4, Field 5] : [Field; 5]
+            v9 = allocate -> &mut [Field; 5]
+            v11 = array_set v8, index v0, value Field 64
+            v13 = add v0, u32 1
+            store v11 at v9
+            v14 = make_array [Field 1, Field 2, Field 3, Field 4, Field 5] : [Field; 5]
+            jmp b1(u32 0)
+          b1(v2: u32):
+            v17 = lt v2, u32 5
+            jmpif v17 then: b3, else: b2
+          b2():
+            v18 = load v9 -> [Field; 5]
+            call f1(v18)
+            return
+          b3():
+            inc_rc v14
+            v20 = allocate -> &mut [Field; 5]
+            v21 = add v1, v2
+            v23 = array_set v14, index v21, value Field 128
+            call f1(v23)
+            v25 = add v2, u32 1
+            jmp b1(v25)
+        }
+        brillig(inline) fn foo f1 {
+          b0(v0: [Field; 5]):
+            return
+        }
+        ";
+
+        let ssa = ssa.loop_invariant_code_motion();
+        assert_normalized_ssa_equals(ssa, expected);
+    }
 }

test_programs/gates_report_brillig_execution.sh

@@ -8,8 +8,6 @@ excluded_dirs=(
   "double_verify_nested_proof" 
   "overlapping_dep_and_mod" 
   "comptime_println"
-  #  Takes a very long time to execute as large loops do not get simplified.
-  "regression_4709"
   #  bit sizes for bigint operation doesn't match up.
   "bigint"
   #  Expected to fail as test asserts on which runtime it is in.

tooling/nargo_cli/build.rs

@@ -42,9 +42,7 @@ fn main() {
 
 /// Some tests are explicitly ignored in brillig due to them failing.
 /// These should be fixed and removed from this list.
-const IGNORED_BRILLIG_TESTS: [&str; 11] = [
-    // Takes a very long time to execute as large loops do not get simplified.
-    "regression_4709",
+const IGNORED_BRILLIG_TESTS: [&str; 10] = [
     // bit sizes for bigint operation doesn't match up.
     "bigint",
     // ICE due to looking for function which doesn't exist.